Shielded protector tube



March 17, 1942. o. ACKERMANN 2,276,638

SHIELDED 'PROTECTOR TUBE Filed April 29, i939 l yTNEssEs; V INVENTORATTORNEY Patentecl Mar. 17, 1942 UNTED S'i'iwifi @FICE Si-HELDEDPROTECTOR TUBE Sylvania Application April 29, 1939, Serial No. 270,814

(Cl. F75- 30) 3 Ciaims.

My invention relates to shielded expulsion discharge-tubes such as areutilized for protecting electrical apparatus against excessive voltages,and more particularly my invention relates to such a tube havingarc-responsive gas evolving material therein for assisting inextinguishing the discharge-arc.

An object of my invention is to provide such a protector-tube having twoelectrodes which extend into the opposite ends of an insulating tube incontact with the bore thereof, said tube being provided with an externalshield. which is insulated from both electrodes, whereby an optimumoperating performance wil1 be obtained, as will be hereinafterdescribed.

A further object of my invention is to utilize a shield consisting oi acoating of aluminum paint which is sprayed onto the outer surface of theinsulating tube; or the shield may be some other equivalentshielding-means consisting of a plurality of layers oi a plurality ofoverlapping at conducting akes or bodies which overlap suiciently toproduce a practically continuous capacitor-effect, while being insulatedfrom each other suiiiciently to malte the shield incapable of conductingcurrent from end to end thereof under the voltage str-ess imposed by thenormal linevoltage, and probably also under the dischargeconditions ofthe device.

With the foregoing and other objects in View, my invention consists inthe structures, elements, combinations and relative dispositions ofparts as hereinafter described and claimed, and illustrated in theaccompanying drawing, wherein- Figure l is a sectional view of a simpleshielded tube utilizing my invention,

Fig. 2 is an enlarged diagrammatic view illus-` trative of thefunctional operation of the shield, and

Fig. 3 is a sectional view of a more elaborate tube embodying myinvention.

In Figure 1, my invention is applied to a prom tector-tube comprising aninsulating tube l of iibre or other arc-responsive gas-evolving materialwhich has the property of evolving a large quantity of substantiallynon-ionized gas, or equivalent small-particulate matter, in the presuence of an arc. The tube shown in Fig. 1 is symmetrical, both ends beingalike, and both ends being furnished with metal electrodes eachconsisting of an outer ferrule 2 and an inner tubular portion 3 whichprojects into the insulating tube l. In accordance with my invention,the tube of Fig. l is provided with an outer shield 4 in the form of acoating of aluminum paint which is sprayed onto the central portion ofthe cuter surface of the tube, so as to form a closely adheringcylindrical sleeve, the ends of which are spaced from both of the outerierrules 2 of the terminal electrodes.

The essential or most important feature of the aluminum-paint shield isillustrated in Fig. 2, which diagraminatically represents an enlargementor a portion of the shield, showing it to consist of a plurality oflayers of overlapping and unconnected metal or conducting flakes 5, theoverlapping ends of which are close enough together to have a verystrong capacitor-effect be tween the overlapping iialres, so that theshield operates, electrostatically, very much as if it were a continuousconducting member, whereas, from a conducting standpoint, the separateoverlapping ales or conducting elements are sufliciently well insulatedfrom each other, in general, so that the shield is incapable of actuallyconducting current frorn end to end thereof. In other words, when a lowvoltage is applied to the shield, from end to end thereof, or even whena fairly high surge-voltage is applied thereto, no current is conductedfrom end to end of the tube, the only current obtained being ofpractically the same order as the current which is obtained when voltageis applied to an insulator. rihis insulating property of the shield isextremely useful in increasing the ability of the tube to avoid externalcurrent-leakage or flashover.

My shielded tube, in common with all other successful internal-dischargetubes, must have an external i'iashover-voltage which is higher than theinternal flashover-voltage, so that a flash-- over or arc always occurson the inside, where it may readily be extinguished. In many designs,notably tubes which are not utilized in conjunction with an externalseries gap device, it is quite desirable for the externalcreepage-distance, along the surface of the insulation, or from theouter ferrule 2 of one electrode to the outer ferrule Z of the otherelectrode, in Fig. l, to be greater than the internal discharge-spacingbetween the inwardly projecting portions 3 of the respective electrodes.In order to afford an adequate factor of safety, I prefer, in suchdesigns, to cause the shield 4 to be separated from both of theelectrode-ferrules 2 by creepage-distances, the sum of which is greaterthan the spacing separating the electrodes 3 within the bore of thetube.

An important feature of my invention is that the terminal electrodes 3,which extend into the respective ends of the insulating tube I,terminate at a point or points in contact with the bore Ii of the tube,as indicated at I in Fig. 1. This feature has a double advantage in thatit brings the tip of the electrode, at 'I, as close as possible to theadjacent end of the shield 4, since both the shield and the electrodeare in contact with the respective walls of the insulating tube I. Inaddition to this advantage, the fact that the inwardly projectingelectrode 3 terminates in contact with he inner surface of theinsulating tube I (and it is suflicient if this contact is a very lightcontact, not necessarily anything approach* ing a perfect contact),contributes materially to the electrical performance oi the tube, byreducing the internal breakdown-voltage thereof, because .so-calledsurface discharges start more easily, as at the contacting surfaces atI, than discharges which must pass through a free air space.

In general, the length of my shield il, as measured along the length ofthe tube, is less than the internal arcing-distance or separationbetween the inwardly projecting terminal-electrodes 3. This feature,again, has several advantages, because, after the shield has beenlengthened so as to extend as far, on the outside of the tube, as theinternal arcing-distance on the inside, any further extension of theshield, so as to overlap a portion of the inwardly projecting electrodeat either end, will have little or no advantage from the standpoint ofreducing the internal flashovervoltage,. while it will have a verydistinct disadvantage in increasing the danger' of obtaining an externalflashover rather than an internal flashover, that is, an externalflashover at a breakdown-voltage which is lower than the internalbreakdown-voltage. And aside from the external flashover-voltage, it isextremely desirable to keep down the external surface-creepage orlealiage distance, as previously explained.

The operative effect of my shield is to cause practically all of theapplied voltage, prior to a breakdown or internal flashover of the tube,to be concentrated in the short spaces, measured through the walls ofthe tube, between the respective ends 'l of the electrodes and theadjacent ends of the shield li, so that the voltage-gradient, or stressper centimeter, which appears on the arcing-tips i of the electrodes, ismuch greater than it would have been if the shield il had been omittedfrom the tube.

3 shows a more elaborate form of embodiment of my invention, in whichthe fibre tube I is made in the form of a plurality of layers 8, Si and2U of telescoping fibre or other insulating tubes of various lengths.The upper electrode in Fig. 3 is in the form of a hollow metal shell I2,which is inserted in the upper end of the outer insulating tube S, andwhich terminates in an arcing-tip i3 which is flush with the innersurface or bore of the inner insulating tube or spacer Iii. The upperelectrode I2 is connected, by means of a threaded steel stud I4, to anupper terminal-member i5 which is connected to a line-conductor it. Thebottom electrode in Fig. 3 is a massive metal casting I'I, which has atubular portion yIii projecting up into the insulating tube andsimilarly terminating flush with the inner bore thereof. The bottomportion of the lower electrode I is also provided with a lateral vent oropening I8 which communicates with the bore of the tubularelectrode-portion I8, so as to provide a means for venting the tube. Thebottom electrode I1 is also provided with a terminal-member 2l whichengages a ground-conductor 22. The outer insulating layer 8 of theinsulating tube I is provided with a shield 4 similar to that which hasbeen described in connection with Fig. 1.

The particular tube which is illustrated in Fig. 3 has a more elaborateinternal structure and a more elaborate external structure than thesimple tube which is shown in Fig. 1. The internal structure consists ofa plurality of slotted diffuser elements or plugs 24, 25 and 2S, whichare spaced by the inner spacers or layers I0 of the compositeinsulating-tube structure, these so-called diffuser plugs 24, 25 and 26being built up of pieces of fibre which are slotted, as indicated bydotted lines 28, so as to provide a plurality of arc-discharge pathswhich are in very close or intimate contact with the walls of thegas-evolving fibre, as described and claimed more explicitly in a patentof Hodnette et al., No. 2,147,440, granted February 14, 1939, andassigned to the Westinghouse Electric & Manufacturing Company.

An external porcelain housing-member 30 is also provided, for the tubewhich is shown in Fig. 3, said porcelain housing-member being providedwith a supporting-clamp 3| which terminates in a supporting-bracket 32,so that the shield I4 is well insulated from the supporting bracket 32,and the entire tubular member I is well pi'otected against the weather.

The operation of the tube shown in Fig. 3 is very similar to that whichhas been described in connection with Fig. 1.

In both of the embodiments of my invention, shown in Figs. l and 3,respectively, the entire outer surface of the insulating tube I, afterthe shielding coating 4 has been applied, is usually covered with aprotective coating of an insulating paint, which covers both the shieldand the tube, and which further serves to insulatingly protect theshield, while at the same time improving the physical appearance of thedevice. This outer paint-coating is too thin to be shown on the drawing,other than as the outer surfaceline which is indicated at 34. In fact,the aluminum-paint coating 4, which constitutes the shield, is also toothin to be properly shown on the drawing, its thickness being grosslyexaggerated for the sake of illustration.

While I have illustrated my shield as consisting of a sprayed-on coat ofaluminum-paint, I intend that this showing, and in particular theenlarged illustration in Fig. 2, shall be regarded as being symbolic orrepresentative of any substantially functionally equivalentshieldingmember which may consist either of small metallic flakes, as inmetal paints, or of overlapping larger shielding-elements of much largeroverlapping and insulated conducting flakes, sheets or pieces, or othermeans for causing the shield to have a capacitor-effect substantially asif it were a solid conducting sheet, whereas it is actually incapable ofconducting current from end to end thereof.

I desire my illustrations of my invention to be construed as beingmerely illustrations and as not indicating that my invention isspecifically limited to any precise structural combination other than isnecessarily inferred from the language of the appended claims, when readin the light of the prior art.

I claim as my invention:

1. A shielded expulsion discharge tube comprising a tubular insulatinghousing-structure, two terminal electrodes at the opposite ends of thetube, respectively, at least one of said electrodes projecting into itsend of said tubular insulating housing-structure and terminating at apoint or points in contact with the bore of said tubular insulatinghousing-structure, at least one of said electrodes being vented to theoutside of the tube, the internal discharge-space or spaces between thetwo electrodes within the bore of the tubular insulatinghousing-structure including arc-responsive gas-evolving insulatingmaterial, a shield in contact with the outside surface of said tubularinsulating housing-structure, said shield comprising a plurality oflayers of a plurality of overlapping flat conducting bodies which aresuciently well insulated from each other to make the shield incapable ofconducting current from end to end thereof, said shield being disposedin non-overlapping relation with respect to both of the electrodes andextending from a point near the arcing-tip of one electrode to a pointnear the arcing-tip of the other electrode, the extent of the projectionof the respective electrode or electrodes into the tubular insulatinghousing-structure being sufliciently great so that the shield isinsulatedly separated from both oi said electrodes, externally of thetube, by creepage-distances the sum of which is greater than the eectivespacing separating the arcing-tips of the electrodes within the bore ofthe tubular insulating housing-structure, and means for so supportingthe tube that said shield is electrically insulated from ground.

2. A shielded expulsion discharge tube cornprising a tubular insulatinghousing-structure, two terminal electrodes at the opposite ends of thetube, respectively, at least one of said electrodes projecting into itsend of said tubular insulating housing-structure and terminating at apoint or points in contact with the bore of said tubular insulatinghousing-structure, at least one of said electrodes being vented to theoutside of the tube, the internal discharge-space or spaces between thetwo electrodes within the bore of the tubular insulatinghousing-structure including arc-responsive gas-evolving insulatingmaterial, a shield in contact with the outside surface of said tubularinsulating housing-structure, said shield comprising a composite sheetof a plurality of materials of such nature and composition that theshield has a capacitor-effect substantially as if it were a solidconducting sheet, whereas it is actually incapable of conducting currentfrom end to end thereof, said shield being disposed in non-overlappingrelation with respect to both of the electrodes and extending from apoint near the arcing-tip of one electrode to a point near thearcing-tip of the other electrode, the extent of the projection of therespective electrode or electrodes into the tubular insulatinghousing-structure being suiiciently great so that the shield isinsulatedly separated from both of said electrodes, externally of thetube, by creepage-distances the sum of which is greater than theeffective spacing separating the arcing-tips of the electrodes withinthe bore of the tubular insulating housing-structure, and means for sosupporting the tube that said shield is electrically insulated fromground.

3. A shielded expulsion discharge tube comprising a tubular insulatinghousing-structure, two terminal electrodes at the opposite ends of thetube, respectively, at least one of said electrodes projecting into itsend of said tubular insulating housing-structure and terminating at apoint or points in contact with the bore of said tubular insulatinghousing-structure, at least one of said electrodes being Vented to theoutside of the tube, the internal discharge-space or spaces between thetwo electrodes within the bore of the tubular insulatinghousing-structure including arc-responsive gas-evolving insulatingmaterial, a shield in contact with the outside surface of said tubularinsulating housing-structure, said shield comprising a coating of paintcomprising a large number of metal flakes arranged in at least partiallyoverlapping layers, with the akes suiiiciently well insulated from eachother to make the shield incapable of conducting current from end to endthereof, said shield being disposed in non-overlapping relation withrespect to both of the electrodes and extending from a point near thearcing-tip of one electrode to a point near the arcing-tip of the otherelectrode, the extent of the projection of the respective electrode orelectrodes into the tubular insulating housing-structure beingsuiiciently great so that the shield is insulatedly separated from bothof said electrodes, externally of the tube, by creepage-distances thesum of which is greater than the effective spacing separating thearcing-tips of the electrodes within the bore of the tubular insulatinghousing-structure, and means for so supporting the tube that said shieldis electrically insulated from ground.

OTTO ACKERMANN.

